The present invention relates to an oxide superconductor which is capable of trapping a high magnetic field and maintaining its performance for a long period of time without being affected by internal or external forces such as an electromagnetic force or thermal stress or by corrosive environments, and also to a process for producing said oxide superconductor.
Regarding a superconducting material, which has a high critical current density as compared with an ordinary conducting material and is capable of passing a large electric current without any loss, research and development have vigorously been conventionally carried out on its application in the field of experimental equipment for nuclear fusion, superconductive MRI for medical diagnosis, magnetic levitation trains, electric generators, energy storage units, magnetometers and the like, and since there have been found metal oxide superconducting materials having a relatively high critical temperature (T) such as LiTi2O3, Ba(Bi,Pb)O3 and (Ba,K)BiO3 in recent years and there have successively been created copper oxide superconducting materials having such a relatively high (T) that had never been anticipated before, such as (La,Sr)2CuO4, REBa2Cu3O7 (RE is a rare earth element), Bi2Sr2Ca2Cu3O10, Ti2Ba2Ca2Cu3O10 and HgBa2Ca2Cu3O8, the research has been spurred more.
A superconducting material has a high critical current density as compared with an ordinary conducting material, and thus is capable of passing a large electric current without any loss. However, it is known that in the case of passing such a large electric current, a material is sometimes destroyed, depending upon its strength, since a large electromagnetic force acts on the superconductor in question.
Accompanying the enhanced characteristics and large scale operation of relatively high temperature bulk superconductors (particularly, copper oxide superconductor), the magnitude of a magnetic field capable of being trapped in a bulk superconductor has recently been drastically enhanced, for instance, to the extent that a magnetic flux density exceeding 5 tesla (T) has come to be trapped (refer to xe2x80x9cSuperconductor Science and Technologyxe2x80x9d 11, 1998, pp. 1345 to 1347). Since the electromagnetic force applied to a superconductor increases with an increase in a trapped magnetic field, there has recently been brought about such a situation in that a restriction is imposed on a trapped magnetic field depending upon a material""s strength. Under such circumstances, importance is attached to an improvement in mechanical properties rather than a further improvement in superconducting properties (refer to xe2x80x9cPhysica Cxe2x80x9d vol. 7, No. 9, 1991, pp. 4989 to 4994).
It being so, the following two proposals have been made as a means for reinforcing an oxide bulk superconductor.
One proposal includes a method in which Ag is added to a material in question. It is said that by taking such a measure, a remarkable improvement is brought about in the mechanical strength of an oxide bulk superconductor (refer to xe2x80x9cJapanese Journal of Applied Physicsxe2x80x9d vol. 70, No. 9, 1991, pp. 4989 to 4994 and xe2x80x9cSuperconductor Science and Technologyxe2x80x9d 11, 1998, pp. 1345 to 1347).
The other proposal includes a method in which a compression strain is applied in advance to a material in question by fitting a bulk superconducting material with a metallic ring (refer to xe2x80x9cExtended Abstract of ISTEC International Workshopxe2x80x9d 1998, pp. 115 to 118). It is said that by taking such a measure, an improvement is brought about in the trapped magnetic field, since the tensile stress caused at the time of trapping the magnetic field is alleviated by the compression strain which was applied in advance, thereby suppressing the destruction of the material.
Nevertheless, the above-mentioned methods including the reinforcement with Ag addition and reinforcement with a metallic ring are desired to make further improvements in the aspects of workability and manufacturing cost. Moreover, the problem has been recognized in that the reinforced performance is deteriorated by long-term use under a corrosive environment.
To solve the problem described above, the inventors have researched repeatedly a method for readily providing at a low cost, an oxide superconductor which is capable of sufficiently withstanding internal or external forces, such as a large electromagnetic force or a thermal stress accompanying a sudden rise or drop in temperature at the time of use, and further capable of exhibiting a high trapped magnetic field for a long period of time without being adversely influenced by a corrosive environment and the findings as described as a)-f) hereinafter have been obtained at the midpoint of the research.
Other objects of the present invention will be obvious from the text of this specification hereinafter disclosed.
In these circumstances, intensive research and investigation were accumulated by the present inventors in order to achieve the above-mentioned objects. As a result, novel information and findings as described hereunder have been obtained.
(a) An oxide bulk superconductor is a ceramic in the state of pseudo-single crystal. It is difficult in practice to prevent microcracks or micropores from being internally included during the manufacture thereof.
(b) When such an oxide bulk superconductor is subjected to xe2x80x9ca strong mechanical impactxe2x80x9d, xe2x80x9cthermal impact due to sudden temperature variationxe2x80x9d, xe2x80x9ca large electromagnetic force (Lorentz force)xe2x80x9d or the like, a stress concentration occurs in the aforesaid microcracks or whereby the microcracks or micropores as starting points progress and expand to relatively large cracks.
(c) In the case where the oxide bulk superconductor is exposed for a long time in a corrosive atmosphere containing a large amount of moisture or carbon dioxide gas, the materials for the oxide bulk superconductor deteriorate, or a reaction phase is formed resulting in the generation of new cracks, which progress and expand to relatively large cracks.
(d) The aforesaid relatively large cracks, when being formed, inhibit the flow of the superconductive current, thus greatly decreasing the trapped magnetic field.
(e) However, even if an oxide bulk superconductor is one which has been believed that there is by no means any possibility of internal permeation of a coating material or the like because of an extremely high density due to its generally being produced by a melting method, contact with a resin in liquid form in an atmosphere of reduced pressure enables said superconductor to maintain a high trapped magnetic field. This is due to the mechanism that the resin permeates not only into the microcracks having openings on the surface, but also into the whole surface layer and, further, the inside of the bulk body through the microcracks having openings, whereby the corrosion resistance of the surface is markedly improved and besides, the mechanical strength of the bulk superconductor itself is drastically enhanced, thereby suppressing, to the utmost, internal forces, external stresses and the propagation of cracking due to corrosion.
(f) In addition, since there is not recognized at all a deterioration, due to the resin impregnation, of the superconductivity characteristics of the bulk body matrix, the above-mentioned method is an extremely advantageous means for improving the mechanical properties and corrosion resistance, while maintaining the excellent superconductivity characteristics of the oxide superconductor.
Such being the case, on the basis of the above-mentioned information and findings, the present inventors previously proposed an oxide superconductor which comprises an oxide superconductive bulk body (for instance, a copper oxide superconductive bulk body containing at least one rare earth element) having a resin-impregnated layer, as an oxide superconductor which is minimized in the generation of cracks due to an external force or internal stress, is hardly adversely influenced by a corrosive environment, and is capable of maintaining a high trapped magnetic field for a long period of time; and further proposed a process for producing the above mentioned oxide superconductor which comprises impregnating a resin into an oxide superconductive bulk body by bringing the resin in liquid form into contact with the bulk body preserved in an atmosphere of reduced pressure {(refer to Japanese Patent Application Laid-Open No. 361722/1988 (Heisei 10)}.
However, by the research and investigations continued thereafter by the present inventors, it has been clarified that an oxide superconductor comprising an oxide superconductive bulk body having a resin-impregnated layer relating to the previous proposal still involves a problem to be further improved as described hereunder.
That is to say, even an oxide superconductor which comprises an oxide superconductive bulk body having a resin-impregnated layer, and which exhibits a markedly high cracking resistance against mechanical shock and thermal stress, sometimes fails as the case may be, to sufficiently exert the expected working effect which is obtained by providing the resin-impregnated layer, and which is shown by the prevention of cracking and corrosion of the oxide superconductive bulk body, since microcracks are generated in the resin-impregnated layer during the time zone soon after cooling in the case where the superconductor is rapidly cooled to the critical temperature or lower.
The object of the present invention, taking into consideration the foregoing, is to establish a method for readily providing at a low cost, an oxide superconductor which is capable of further sufficiently eliminating the risk of crack generation due to a mechanical strain derived from a large electromagnetic force or due to a thermal strain accompanying a sudden rise or drop in temperature at the time of use, and at the same time, which is capable of maintaining a high trapped magnetic field for a long period of time whether under an ordinary environment or a corrosive environment.
In these circumstances, intensive research and investigation were performed by the present inventors in order to achieve the above-mentioned object. As a result, the facts as described in the following items (a) to (g) were elucidated.
(a) The previous proposal, that is, the generation of microcracks which is recognized as the case may be, in the resin-impregnated layer at the time of rapidly cooling the oxide superconductor having a resin-impregnated layer, is a phenomenon originating from the difference in thermal expansion coefficient (thermal contraction coefficient) between the oxide superconductive bulk body and the resin-impregnated layer. The resin-impregnated layer, which has a linear thermal expansion coefficient higher than that of the oxide superconductive bulk body, contracts to a greater extent at the time of cooling with the more possible result that said layer can no longer withstand the tensile stress arising therefrom, thus leading to final cracking. Therefore, it is indispensable to solve such a problem in order to sufficiently assure the effect on preventing the cracking.
(b) However the linear thermal expansion coefficient of said resin-impregnated layer can be lowered by dispersedly incorporating therein a filler material which has a low value of linear thermal expansion coefficient and is exemplified by quartz, calcium carbonate, alumina and glass. Thus by properly selecting the combination of the type and blending amount of a filler material, it is possible to realize a resin material having a linear thermal expansion coefficient almost the same as that of the oxide superconductive bulk body.
(c) It being so, when a resin layer dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient is formed on the outside surface of the oxide superconductor having a resin-impregnated layer as previously proposed, then said resin-impregnated layer is brought to a state wherein it is interposed between the oxide superconductive bulk body having a low value of linear thermal expansion coefficient and the resin layer which contains the filler material and has a low value of linear thermal expansion coefficient. Accordingly, the tensile stress generated at the time of rapid cooling is held down by the above two, thus suppressing the generation of cracking.
(d) In addition, when a resin dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient is directly impregnated into the oxide superconductive bulk body so that said bulk body is equipped with the resin-impregnated layer dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient, then, as is the case with the item (c), cracking or corrosion is no longer generated, thereby realizing an oxide superconductor capable of exhibiting a high trapped magnetic field for a long period of time.
(e) However, because of its increased viscosity, the resin dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient is difficult to form into a layer deeply impregnated into the oxide superconductive bulk body. In such a case, sufficient cracking and corrosion resistance can be imparted to an oxide superconductor by further covering the outside surface of the oxide superconductive bulk body having a resin-impregnated layer dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient with a resin dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient.
(f) On the one hand, relatively large cracks that are prone to be generated on the oxide superconductive bulk body with the lapse of time can be effectively prevented by wrapping and covering the outside surface thereof with woven or non-woven fabric of fibers (glass fiber, carbon fiber, ceramic fiber, metal fiber, polyamide-based synthetic high polymeric fiber, cotton fiber, silk fiber, wool fiber, etc.), and thereafter impregnating the fabric with a resin to form a layer covered with resin-impregnated fabric. The oxide superconductive bulk body, even when equipped with such a layer covered with resin-impregnated fabric, is entirely free from the deterioration in superconductive properties. By simultaneously employing the resin impregnation into the bulk body along with the formation of the layer covered with resin-impregnated fabric, it is made possible to more steadily suppress the propagation of cracking derived from an external force, internal stress or corrosion at the time of use.
(g) In the case of simultaneously employing the resin impregnation into the bulk body along with the formation of a layer covered with a resin-impregnated fabric, by dispersedly incorporating a filler material having a low value of linear thermal expansion coefficient, such as quartz, calcium carbonate, alumina and glass, in a resin material to be impregnated into the inside of the oxide superconductive bulk body, it is made possible to lower the linear thermal expansion coefficient of said resin material; and further by properly selecting the combination of the type and blending amount of the filler material, there is materialized a resin material having a linear thermal expansion coefficient almost the same as that of the oxide superconductive bulk body. In this case, by constituting the resin-impregnated layer to be formed inside the oxide superconductive bulk body, the impregnated resin layer dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient, it is made possible to completely clear the concern about the cracking of the resin-impregnated layer, the generation of which could not have been completely denied, said cracking being due to the difference in thermal expansion (contraction) coefficient between the resin-impregnated layer and the oxide superconductive bulk body. As a result, the above-mentioned advantage leads to a further enhancement of the stability of properties as well as reliability of the oxide superconductor.
The present invention, which has been accomplished on the basis of the aforesaid information and findings, provides the under-mentioned oxide superconductor and also a process for producing said oxide superconductor.
(1) An oxide superconductor comprising an oxide superconductive bulk body which has a resin-impregnated layer, and the outside surface of which is covered with a resin layer dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient.
(2) An oxide superconductor comprising an oxide superconductive bulk body which has on a surface portion thereof, a resin-impregnated layer dispersedly incorporated with a filler material having a low value of linear thermal expansion coefficient.
(3) An oxide superconductor comprising an oxide superconductive bulk body which has on a surface portion thereof, a resin-impregnated layer dispersedly incorporated with a filler material having a low value of linear thermal expansion coefficient, and the outside surface of which is covered with a resin layer dispersedly incorporated with the above-mentioned filler material having a low value of linear thermal expansion coefficient.
(4) The oxide superconductor as set forth in any of the preceding items, wherein the resin in the resin-impregnated layer comprises an epoxy base resin.
(5) The oxide superconductor as set forth in any of the preceding items, wherein the filler material is at least one member selected from the group consisting of quartz, calcium carbonate, alumina, hydrate alumina, glass, talc and calcined gypsum.
(6) A process for producing the oxide superconductor as set forth in any of the items (1), (4) and (5) which comprises impregnating a resin into an oxide superconductive bulk body by bringing the resin in liquid form and said bulk body preserved in an atmosphere of reduced pressure into contact with each other, and thereafter coating the resin-impregnated bulk body with a resin in liquid form, which is dispersedly incorporated with a filler material having a low value of linear thermal expansion coefficient.
(7) A process for producing the oxide superconductor as set forth in any of the items (2) and (5) which comprises impregnating a resin into an oxide superconductive bulk body by bringing the resin in liquid form, which is dispersedly incorporated with a filler material having a low value of linear thermal expansion, and said bulk body preserved in an atmosphere of reduced pressure into contact with each other.
(8) A process for producing the oxide superconductor as set forth in any of the items (2) and (5) which comprises impregnating a resin into an oxide superconductive bulk body by bringing the resin in liquid form, which is dispersedly incorporated with a filler material having a low value of linear thermal expansion coefficient, and said bulk body preserved in an atmosphere of reduced pressure into contact with each other, and thereafter coating the resin-impregnated bulk body with a resin in liquid form, which is dispersedly incorporated with a filler material having a low value of linear thermal expansion coefficient.
(9) An oxide superconductor comprising an oxide superconductive bulk body which has an adhesively covering layer of resin-impregnated fabric on the outside surface.
(10) The oxide superconductor as set forth in item (9), wherein the fabric constituting said adhesively covering layer comprises fibers selected from the group consisting of glass fiber, carbon fiber, ceramic fiber, metal fiber and polyamide-based synthetic high polymeric fiber.
(11) The oxide superconductor as set forth in item (9) or (10), wherein the oxide superconductive bulk body has on the surface portion thereof, a resin-impregnated layer.
(12) The oxide superconductor as set forth in any of the items (9) through (11), wherein the resin in the resin-impregnated layer comprises an epoxy base resin.
(13) The oxide superconductor as set forth in any of the items (9) through (12), wherein the resin is dispersedly incorporated with a filler material having a low value of linear thermal expansion coefficient.
(14) A process for producing the oxide superconductor as set forth in any of the items (9) through (13), which comprises wrapping the surface of the oxide superconductive bulk body, and thereafter bringing the wrapped body preserved in an atmosphere of reduced pressure into contact with a resin in liquid form.